The present invention relates generally to phase sensing of alternating currents in a polyphase distribution system, and more particularly, to a method and apparatus for three phase sensing using a separate burden for each current transformer.
Current transformers (CTs) are used for sensing AC electrical currents in load control and in protection devices. For example, CTs are used in sensing electrical currents through contactors, motor starters and controllers, circuit breakers, monitors and analyzers, and in general, electrical distribution systems. In many such applications, these products are polyphase, or more particularly three-phase, and generally require a CT for each phase.
Most modern prior art attempts at monitoring overload and fault conditions in a load supplied by a multiphase, or polyphase power supply, use a current transformer for each separate phase of the three-phase power distribution system. For example, U.S. Pat. No. 4,967,304 discloses a digital circuit interrupter applicable for use on a three-phase power distribution system wherein a separate current transformer is required for each separate phase of the distribution system. One attempt at using two current transformers to detect phase failure and overload is disclosed in U.S. Pat. No. 2,202,998. However, the two CTs monitor only two of the three phases, and the third phase is only indirectly monitored. That is, a failure or overload on the unmonitored third phase is detected by the reaction it has on the two monitored phases. The third phase itself is not monitored. Further, a phase loss in the unmonitored phase will go undetected until the two CTs detect the resulting higher currents in the two monitored phases, which may be too late to protect modern loads having very tight thermal tolerances.
In monitoring the secondary windings of the CTs, the prior art has commonly used a single burden resistor for combining and averaging a representation value for the currents in each phase. In such systems, it is not possible to determine the true RMS value for each current in each phase. By combining the currents in a lump sum in a single burden, it is not possible to detect an out of balance condition in a load without monitoring some other characteristic. In other words, in a three phase load drawing 1 amp RMS in each of the three phases, a single burden resistor will provide a 2.7 amp average signal under a normal, balanced operation. However, in an out of balance condition where the load is drawing 1 amp in one phase, 0.5 amps in a second phase, and 1.5 amps in a third phase, the single burden system has no way of detecting that one phase is three times higher than the other and the single burden resistor will continue to provide a 2.7 amp average signal indicating a balanced condition.
Therefore, in such systems it is necessary to monitor some other condition, such as voltage ripple on burden, which merely gives an approximation of the significant difference in currents, and adds complexity and expense to such systems.
One attempt at approximating an RMS current for each phase is disclosed in U.S. Pat. No. 5,450,268 for which is said to approximate the true RMS value within plus or minus 5%. This system approximates the RMS line current by determining a peak value of the current and combining it with a determined average value of the current in each phase. However, in this system since the negative side of each rectifier is connected to ground and the burden is taken with reference to the positive side of the rectifier, it is unable to provide a DC voltage supply to self-power the circuit. Further, this system does not provide a signal capable of providing the true RMS, but merely an approximation of the true RMS signal according to empirical data.
Other prior art attempts at tracking RMS current values include placing a single burden resistor in the return or common path to produce a signal proportional to the sum of the three phase currents so as to avoid having to consider the current drawn by the circuit itself. The problem with such a configuration is that the contributions of individual phases is unknown. For example, if three CTs each contributed a 1 amp RMS current, the total current in a single collection resistor would be 2.7 amp average. Conversely, whenever the value in the single collection resistor would be 2.7 avg., the circuit would assume each phase current to be equal to 1 amp RMS and the avg. I.sup.2 total would be 7.3. However, if the contributions were actually 0.5, 1.5, and 1, the sum would still be 2.7, but the actual avg. I.sup.2 total would 7.5. This represents only a 3% increase in the I.sup.2 total, yet one phase is operating at 150% current. Therefore, single burden systems merely average the I.sup.2 total and are often in error and require some other means for determining out of phase or phase loss conditions.
In a self-powered application, the circuit must not only generate a voltage to operate the circuit, but also a signal that accurately represents the actual current to be measured. Therefore, it would desirable to have a method and apparatus for self-powered three phase sensing capable of determining true RMS current values that solves the aforementioned problems. It would be further advantageous to provide such a method and apparatus with only two current transformers.